It is well-known that the size of magnetic components can be decreased by increasing the operating frequency. However, as frequency is increased, winding losses increase due the presence of eddy currents in the conductors. These eddy currents are caused by AC effects which are magnified at high frequencies, such as skin and proximity effects and fringing fields from air gaps.
Conventional windings at low frequencies are generally solenoidal or helical and are made from circular, square, or foil conductors. At high frequencies, however, the AC-to-DC resistance ratio of such conductors increases markedly due to skin and proximity effects. Thus, for effective utilization of a conductor cross-section, it is advantageous to constrain one dimension of the conductor to one or two skin depths. Consequently, and in contrast to the low frequency case, planar windings are often employed which assist in minimizing the overall volume of an electrical component designed to carry a specified current at high frequencies. Disadvantageously, in order to carry high current or to exhibit a low resistance characteristic, the other cross-sectional dimension of the planar winding cannot be so constrained. Therefore, although conductor volume efficiency is improved by using planar windings, eddy currents and their attendant losses still persist, and the reduction of such eddy currents is of high concern.
Conventional magnetic structures, such as inductors, have high-permeability cores with lumped air gaps. A conventional core also has a winding window for containing conductors encased by an insulating material. The air gaps in a core of sufficiently small volume are so large relative to the overall window size that the fringing field flux penetrates the conductors. Such field non-uniformity generates excessive eddy current losses. As a result, the AC resistance is significantly larger than the DC resistance.
With reference to FIG. 1, a conventional inductor is shown. A high-permeability core 12 having lumped air gaps 10 includes a winding window 14. The winding window contains planar conductors 16a, 16b, 16c, 16d and 16e encased by an insulating material 18. Referring now to FIG. 2, a graph illustrates the magnetic field intensity tangential to the surfaces of the planar conductors of FIG. 1 as a function of the distance from either side of the core. One of ordinary skill in the art will appreciate that such field non-uniformity generates excessive eddy current losses.
It has been proposed that one way to reduce the AC winding losses, without increasing the size of the winding window, is to distribute the air gaps uniformly around the magnetic core as discussed in "Effects of Air Gaps on Winding Loss in High-Frequency Planar Magnetics" by Khai D. T. Ngo and M. H. Kuo, Power Electronics Specialists Conference Proceedings, Apr. 11-14, 1988, pp. 1112-1119, which is incorporated herein by reference. This distributed gap effect could be realized by constructing the inductor with a magnetic core of ferrite having a low, controllable permeability. The low-permeability core forms a closed-loop structure surrounding the winding window which contains planar copper conductors encased by an insulating material. Although the core structure of low-permeability would reduce the AC winding losses, these losses would still be too high because of the uneven distribution of current in the conductors resulting from field non-uniformity. Specifically, regions of high field intensity result from the crowding of flux lines around corners of the core structure as they follow the paths of least reluctance. This high field intensity causes significant eddy current circulation in the outermost conductors of the winding.
A distributed gap inductor having the characteristics hereinabove described is illustrated in FIG. 3. Low-permeability core 20 includes winding window 22 which contains planar copper conductors 24a, 24b, 24c, 24d and 24e encased by insulating material 26.
Another approach to loss reduction, also discussed in "Effects of Air Gaps on Winding Loss in High-Frequency Planar Magnetics", cited above, is to employ a multi-layer winding in a distributed gap inductor. Use of a multi-layer winding not only improves the aspect ratio of the core geometry, but also results in reduced core losses. Further, an inductor having a multi-layer winding of the same current and frequency rating requires a larger winding window than its single-layer counterpart, the use thereof thus alleviating the adverse effects of field non-uniformity. Unfortunately, despite the above enumerated advantages, the stacking of conductors to form a multi-layer winding causes higher proximity effect losses. The overall result, however, is an inductor having a comparable or a slightly lower AC-to-DC resistance ratio than the single-layer distributed gap inductor.
Although the above-described recent proposals for magnetic core structures result in lower winding losses, these losses and, thus, the AC-to-DC resistance ratios, are still too high for practical purposes. That is, while AC-to-DC resistance ratios greater than five have been achieved, a ratio closer to unity is desirable. The present inventors, therefore, propose the use of a dual-permeability magnetic core structure comprising alternating sections of high- and low-permeability materials. In a rectangular coordinate system, for example a rectangular or "sleeve" core, an optimized configuration of a dual-permeability core structure would result in a highly uniform magnetic field profile about the planar conductor surfaces. As the term is used herein, a sleeve core is defined as a hollow structure of rectangular cross-section. Further, in a cylindrical coordinate system, for example a cylindrical "pot core", an optimized dual-permeability core structure would result in a magnetic field tangential to the planar winding surfaces which varies inversely with its radius. A pot core is defined herein as a hollow, cylindrical structure having an interior concentric core post. In developing dual-permeability core structures, the present inventors have overcome problems of configuration, optimization, and fabrication of magnetic materials of variable permeability.